New antitumor agents and those in clinical use are being evaluated for embryotoxic, cytogenetic and carcinogenic effects. Comparative studies of the embryotoxicity and cytogenetic effects in mice of a series of antitumor agents known to be spindle poisons are in progress. The agents under study are vincristine, maytansine, colchicine and two epipodophyllotoxin derivatives (VP-16 and VM-26). Various antitumor agents, including adriamycin, procarbazine, azathioprine and melphalan are being tested for carcinogenic activity in long-term studies in monkeys. The medical literature as well as patient data at the NCI continues to be monitored in order to gain more information as to the carcinogenic potential in humans of clinically used antitumor agents. Other adverse effects of these agents are also continuously being tabulated from the literature, including teratogenic, mutagenic and cytogenetic effects.